If you want to see global warming in action, head to the Arctic. The seasonal shrinking of the sea ice over the North Pole is one of the most visible symptoms of the gradual warming of the planet. Every winter, Arctic ice builds up in the polar darkness, and then in the summer, it melts. Over the last several years—as temperatures in the Arctic have warmed even faster than they have throughout the rest of the planet—ice has been melting quickly. In September 2007 the Arctic sea ice hit a minimum of 1.608 million sq. mi.—an all-time low since record-keeping began more than 50 years ago. So much sea ice had melted that the fabled Northwest Passage—an Arctic shortcut between the Atlantic and Pacific Oceans—briefly opened up for the first time in human memory.

After the intense heat that 2011 has experienced so far, there has been speculation that this year might witness a new record low for Arctic sea ice. Not quite—but very, very close. On September 15 scientists at the University of Colorado’s National Snow and Ice Data Center (NSIDC) announced that the 2011 Arctic sea ice extent fell to 1.67 million sq. mi., the second-lowest level on record. That’s just a bit more than the record 2007 levels, but what’s even more striking is that this year’s minimum is more than 1 million sq. mi below the 1979-2000 average for September—an area larger than Texas and California combined. Nor is this melt a one-off—the last five years have seen the five lowest Arctic sea-ice extents since satellite measurements began in 1979. And there’s no doubt what’s causing it. “This is not a random event,” says NSIDC oceanographer James Overland. “It’s a long-term change in Arctic climate.”

How much has the ice cap shrunk? Walt Meier, a senior scientist at NSIDC, estimates that an amount of ice the size of all the states east of the Mississippi River and the states just bordering it to the west have disappeared since the 1980s. What ice has remained has grown increasingly thin and weak. There’s also a bit of a feedback process at work here. White ice reflects sunlight, providing a cooling effect, while dark open seawater absorbs it, increasing warming. The more warming, the more melting, and the more melting, the more open seawater and the more warming. Altogether, says Meier, “we haven’t seen ice cover this low for the last several thousand years.”

What’s even more sobering is that 2011 probably missed out on setting the record only because of a few random meteorological factors that have little to do with warming. In 2007, the rate of melting was aided by unusually sunny skies during the summer solstice as well as winds that helped push ice northward, contracting it. “This year, the punch [from other weather factors] wasn’t as strong, but the ice itself was already a lot weaker,” says Meier. “If we had conditions this year like we had in 2007, I think we almost definitely would have gone below 2007 levels.”

It’s worth noting that another group of researchers based at the University of Bremen recently did their own estimate of the 2011 Arctic melt and reported that this year did indeed pass 2007 levels. NSIDC reports are considered authoritative, though, but the question is almost academic—the real point here is that warming is happening and the Arctic is melting. Nor is it likely to stop over the long run. Scientists once estimated that it might take the entire century for the Arctic to finally become ice-free in the summertime. As melting has accelerated in recent years, however, scientists have been revised their predictions for total melt. “If you adjust for the fact that the real world is happening faster than in the models, I think we’re looking at 2030 or 2040″ for an ice-free Arctic,” says Overland.

All this bad news has led some writers to argue that the Arctic has entered a “death spiral“—that enough warming is already built into the climate system, thanks to past carbon emissions, that an ice-free Arctic is inevitable. Others are less certain—Andrew Revkin at Dot Earth has pointed to recent studies showing significant variability in the extent of Arctic ice since the end of the last ice age, which would indicate that there could be a lot of natural variation at work. Another recent study by scientists at the National Center for Atmospheric Research suggests that sea ice melting could actually stall or reverse over the next decade. That pause, predicted by some computer simulations, would be due to changing wind patterns and other temporary factors. Ice is tricky stuff, and it’s hard to forecast just how it will respond to warming.

But over the long-term, there’s little to no doubt that as the climate keeps warming, Arctic sea ice will vanish. That won’t directly affect sea levels, but it will likely accelerate the loss of land ice in places like Greenland, which does. An ice-free Arctic would also change weather patterns and the climate in ways we can’t yet predict. But there’s one thing we do know, as Meier puts it: “What happens in the Arctic doesn’t stay in the Arctic.”

Bryan Walsh is a senior writer at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’sFacebook page and on Twitter at @TIME